Use toolchains/prebuilt/$HOST_TAG/ to compile target prebuilts

Previously we always choose 32-bit $HOST_TAG32 toolchains to compile
target prebuilts, when most developers are on 64-bit host and only
download the 64-bit host NDK.  On 64-bit host, this CL will try
64-bit host toolchains first before falling back to 32-bit host
toolchains

Change-Id: I4eb3f6ce2e56d0a4c79a1cb70a6a47562b8a1a0c
Add testcase for fix to aarch64/arm_aeon.h about FP_LO_REGS

See https://android-review.googlesource.com/#/c/95156

Change-Id: Idf346d51b573eab06221f6d57614180522044039
Merge "Use toolchains/prebuilt/$HOST_TAG/ to compile target prebuilts"
Merge "Add testcase for fix to aarch64/arm_aeon.h about FP_LO_REGS"
Merge branch 'master' of https://android.googlesource.com/platform/ndk into HEAD